(a) Field of the Invention
The present invention relates to a photoalignment agent of a liquid crystal for a liquid crystal display (LCD) and a liquid crystal photoalignment film made using the same. More particularly, the present invention relates to a photoalignment agent of a liquid crystal having a long life-span, being capable of stably maintaining a pretilt angle, having improved after-image characteristics, and having excellent liquid crystal alignment properties and chemical resistance.
(b) Description of the Related Art
A liquid crystal display (LCD) includes a liquid crystal alignment layer. The liquid crystal alignment film is mainly made of polymer materials. The liquid crystal alignment film plays a role of a director in aligning liquid crystal molecules. When the liquid crystal molecules are moved by the influence of an electric field to display an image, the liquid crystal alignment film allows them to be oriented in a predetermined direction. Generally, it is necessary to uniformly align the liquid crystal molecules in order to provide uniform luminance and a high contrast ratio to the liquid crystal device.
The conventional method of aligning the liquid crystal includes coating a polymer film such as a polyimide on a substrate made of a material such as glass, and rubbing the surface of the substrate with a fiber such as nylon or polyester in a certain direction. However, the rubbing method may cause serious problems while fabricating a liquid crystal panel due to fine dust or electrostatic discharge (ESD) that may be generated while rubbing the polymer film with the fiber.
In order to solve the problems of the rubbing method, the photo-radiation method has recently been researched to induce anisotropy to the polymer film by irradiating light on the membrane so as to align the liquid crystal molecules.
As polymer film materials for the photoalignment method, polymers having photo-functional groups such as azobenzene, cumarin, chalcone, and cinnamate have been suggested. Such polymers are anisotropically photo-isomerized or photo-cross-linked by being irradiated with polarized light, so as to provide anisotropy to the surface so that it can induce the liquid crystal molecules to align in a certain direction.
The material for the liquid crystal alignment film should have optical stability and thermal stability, as well as no after-image in order to apply it a substantial liquid crystal display device (LCD). However, the conventional photoalignment materials have many troubles in this respect.
Further, the conventional material for the liquid crystal photoalignment film is mainly polymeric that has a main chain of a polymer and a side group of a photo-functional group that is capable of inducing the photo-anisotropy, such as azobenzene or cinnamate. When the material for a polymeric liquid crystal photoalignment film is used, it may cause problems not only in that a lot of photo energy is required to induce the anisotropy, but also that the thermal stability, optical stability, and electro-optical characteristics are seriously affected by a plurality of unreacted remaining photo-functional groups.